In a digital cellular radio system which operates in accordance with the TDMA-principle, radio messages are transmitted in frames from a radio transmitter, for instance a base station, each frame including a given number of time slots. In this respect, it is necessary to synchronize the radio receiver (the mobile) with the radio transmitter for the time slot intended for the radio receiver. This synchronization must be effected quickly and independently of the time dispersion of the radio medium due to multipath propagation and fading. Furthermore, synchronization must be effected independently of the frequency errors that can occur during radio transmission.
It is known to compensate for the time dispersion of the radio medium, by coherently correlating the received and demodulated radio signal; see for instance Swedish Patent Application 8902844-3 corresponding to U.S. Pat. No. 5,230,079. According to this patent application, the received and demodulated radio signal is coherently correlated with the intention of setting or adjusting an equalizer which functions to compensate for or to equalize the echoes that are obtained with multipath propagation. As described in the aforesaid patent application, and also in WO88/05981, each received time slot includes a synchronizing word which is used to activate and set the equalizer. The synchronizing word is used to enable the equalizer to carry out its function at the correct time position in relation to the time slots in the received radio signal. The equalizer is normally included in the demodulator for the baseband signal, which may be phase-shift modulated (QPSK) for instance. If no equalizer is included in the demodulator, the demodulator still requires synchronization signals in order to carry out its function.